1. Field of the Invention
The present invention generally relates to tracking control methods and storage units, and more particularly to a tracking control method which carries out a tracking control by following a change in a light emission power of a light source and to a storage unit such as an optical disk unit which employs such a tracking control method.
2. Description of the Related Art
In optical disk units which record and/or reproduce information with respect to an optical disk such as a magneto-optical disk, a tracking control is carried out so that a light beam spot scans a desired track on the optical disk. The tracking control is sometimes also referred to as a track servo. Normally, the tracking control is carried out based on a tracking error signal (TES) which is obtained by a known method and indicates an error of the light beam spot from the desired track.
When a write/erase process is carried out during the tracking control and a light emission power of a light source increases, a wavelength of the light beam emitted from the light source becomes long, thereby generating a xe2x80x9cTES shiftxe2x80x9d and a xe2x80x9cTES fluctuationxe2x80x9d as will be described later.
A beam splitter always exists in an optical system of the optical disk unit. The beam splitter transmits the light beam from the light source and supplies the light beam to the optical disk via an objective lens, and supplies the light beam reflected from the optical disk to a light detection system. In a case of an optical disk unit having a construction such that the light beam from the light source is incident at an angle with respect to a beam splitter 1 as shown in FIG. 1, a refractive index of the beam splitter 1 changes when the wavelength of the light beam changes. Hence, in this case, a deviation is introduced in an optical axis of the light beam transmitted through the beam splitter 1. If a cross section of the light beam from the light source has an oval shape, the light beam is input to the beam splitter 1 at an incident angle so as to correct the cross section to a true circular shape. When the optical axis deviates as described above, an irradiated position of the optical beam on a photodetector of the light detection system also deviates from a normal position, thereby generating an offset in the TES. As a result, the TES shifts, and the xe2x80x9cTES shiftxe2x80x9d described above is generated.
In FIG. 1, the following relationship stands, where xcex81 denotes an incident angle of the light beam from the light source to the beam splitter 1, xcex82 denotes an angle formed by a normal with respect to an incident surface of a block 1A of the beam splitter 1 and the light beam propagating within the block 1A, xcex83 denotes an incident angle of the light beam from the block 1A to a block 1B of the beam splitter 1, xcex84 denotes an angle formed by a normal with respect to an incident surface of the block 1B and the light beam propagating within the block 1B, xcex85 denotes an angle formed by the light beam propagating within the block 1B and a normal with respect to a light output surface of the block 1B, xcex86 denotes an output angle of the light beam from the beam splitter 1 to an objective lens, xcex7air denotes a refractive index of air, xcex7A denotes a refractive index of the block 1A, and xcex7B denotes a refractive index of the block 1B.
xe2x80x83xcex7airxc2x7sinxcex81=xcex7Axc2x7sinxcex82
xcex7Axc2x7sinxcex83=xcex7Bxc2x7sinxcex84
xcex7Bxc2x7sinxcex85=xcex7airxc2x7sinxcex86
In this case, the incident anglexcex81 will not change. But since the refractive indexes xcex7A and xcex7B of the blocks 1A and 1B change when the wavelength of the light beam changes, the output angle xcex86 is affected by the change in the wavelength.
On the other hand, in the optical disk unit, a focus control is carried out to control a focus position of the light beam, by controlling the objective lens so as to form the light beam spot on the optical disk. As shown in FIG. 2, in an optimum focus state, an objective lens 2 is controlled so that the light beam converges at an optimum focus position on an optical disk 3. However, when the wavelength of the light beam becomes long as described above, a focal distance of the objective lens 2 also becomes long as shown in FIG. 3. As a result, the light beam in this case no longer converges at the optimum focus position on the optical disk 3, and it takes time to converge the light beam at the optimum focus position on the optical disk 3 by controlling the objective lens 2. For this reason, the TES becomes unstable during the time it takes to converge the light beam at the optimum focus position by controlling the objective lens 2, and the xe2x80x9cTES fluctuationxe2x80x9d described above is generated.
When the TES shift and/or the TES fluctuation described above is generated, a control is activated in an opposite direction so as to suppress the TES shift and/or the TES fluctuation, and as a result, unnecessary control and vibration are generated in servo systems. Particularly in a track servo system, there was a problem in that the write performance deteriorates when the TES fluctuation is generated during the write/erase process. In addition, when the TES shift is generated, the light beam scans a position deviated from a center of the track, and not the center of the track, and there was a problem in that the write performance also deteriorates from this point of view. Furthermore, when the vibration of the servo systems is large, an off-track detection sensitivity becomes excessively sensitive, and there was a problem in that the write performance also deteriorates from this point of view.
Accordingly, it is a general object of the present invention to provide a novel and useful tracking control method and a storage unit, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a tracking control method and a storage unit, which suppress effects of instability factors of the track servo system generated when the light emission power of the light source changes, that is, the effects of the TES shift, the TES fluctuation and the like, so as to stabilize the servo systems and to improve the write performance.
Still another object of the present invention is to provide a tracking control method for carrying out a tracking control based on a tracking error signal which indicates an error of a beam spot formed on a recording medium by a light beam from a light source with respect to a track, comprising a detecting step detecting a change in a light emission power of the light source, a calculating step calculating a correction quantity which is used to correct a shift and/or fluctuation of the tracking error signal generated due to the change in the light emission power, and a correcting step carrying out a correction by adding the correction quantity to the tracking error signal when the change in the light emission power is detected by the detecting step. According to the tracking control method of the present invention, it is possible to realize a tracking control method and a storage unit, which suppress effects of instability factors of the track servo system generated when the light emission power of the light source changes, that is, the effects of the TES shift, the TES fluctuation and the like, so as to stabilize the servo systems and to improve the write performance.
A further object of the present invention is to provide a storage unit comprising a light source emitting a light beam, a tracking control section carrying out a tracking control based on a tracking control based on a tracking error signal which indicates an error of a beam spot formed on a recording medium by the light beam with respect to a track, a detecting section detecting a change in a light emission power of the light source, a calculating section calculating a correction quantity which is used to correct a shift and/or fluctuation of the tracking error signal generated due to the change in the light emission power, and a correcting section carrying out a correction by adding the correction quantity to the tracking error signal when the change in the light emission power is detected by the detecting section. According to the storage unit of the present invention, it is possible to realize a tracking control method and a storage unit, which suppress effects of instability factors of the track servo system generated when the light emission power of the light source changes, that is, the effects of the TES shift, the TES fluctuation and the like, so as to stabilize the servo systems and to improve the write performance.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.